The present invention relates to a hydrodynamic bearing in which a dynamic pressure is generated in a lubricating fluid located between a dynamic pressure surface of a shaft member and a dynamic pressure surface of a bearing member, whereby both members are supported so as to be relatively rotatable, and a method of manufacturing the same.
Recently, there are many proposals on a hydrodynamic bearing for rotatably supporting various types of a rotary body rotating at high speed, such as a polygon mirror, a magnetic disc, an optical disc, or the like. In the hydrodynamic bearing, a dynamic pressure surface of a shaft member is disposed facing a dynamic pressure surface of a bearing member (bearing sleeve) when viewed in the radial direction in a state that a predetermined gap is present therebetween. A hydrodynamic bearing portion is formed in the gap. A dynamic pressure generating groove is formed in at least one of those dynamic pressure surfaces. A lubricating fluid that is injected into the hydrodynamic bearing portion, such as air or oil, is pressurized through a pumping action by the dynamic pressure generating groove when it rotates. The shaft member and the bearing member, while being in a floating state, are supported by a dynamic pressure of the lubricating fluid in a state that those are relatively rotatable.
Recently, in various types of the rotary-body drive device employing such a hydrodynamic bearing, the size and thickness reduction thereof rapidly progresses. With this trend, attempt has been made of making the bearing member (bearing sleeve), which supports shaft member, of a copper group metal. The reason for this is that even when the hydrodynamic bearing is reduced in diameter for the purpose of size reduction, easy working of the bearing member, in particular the inner side thereof, is secured by using phosphor bronze or the like, which is good in workability, for the bearing member. Where the bearing member of the copper group metal is used, austenitic stainless steel, such as SUS303 (indication based on JIS), in place of the usually used, austenitic stainless steel, such as SUS420J2 or SUS440C (indication based on JIS), is sometimes used for the shaft member with such an intention that the shaft member has a thermal expansion coefficient comparable with that of the bearing member.
As described above, if the shaft member is made of the austenitic stainless steel, its surface hardness is reduced, because of its properties, when comparing with that of the shaft member made of the martensitic stainless steel, usually used. Accordingly, its hardness cannot be increased even if it is subjected to sintering process. Therefore, in working process of the shaft member, the surface of it is easy to be flawed, and parts protruded around the flaws come in contact with a dynamic pressure surface of the bearing member as a counterpart member. Sometimes, this leads to reduction of device lifetime.
To solve the problem, in the device disclosed in Japanese Patent Publication No. 10-89345A, the shaft member made of the austenitic stainless steel is subjected to a nitriding treatment, whereby a nitrided layer is formed on the surface of the shaft member, and its surface hardness is increased. In the nitriding treatment, the passive coating inherently possessed by the stainless steel is substituted by a nitrided iron. As a result, its corrosion proof is remarkably deteriorated, so that it is easy to be rusted. The nitrided layer formed by the nitriding process is put on a surface layer of a flexible base. Therefore, it is difficult to prevent indentations that will be formed in the shaft member by hitting.
Accordingly, an object of the present invention is to provide a hydrodynamic bearing which improves the surface hardness of the shaft member without deteriorating its corrosion proof, and well prevents the protrusion formation in its surface, and a method of manufacturing the same.
To achieve the above object, there is a hydrodynamic bearing comprising:
a shaft member having a first dynamic pressure surface;
a bearing member having a second dynamic surface facing the first dynamic pressure surface while having a gap therebetween, which is held rotatably relative to the shaft member and made of a copper group metal containing a copper component;
lubricating fluid being interposing between the gap between the first and second dynamic pressure surfaces; and
a plurality of dynamic pressure generating grooves formed in at least one of the first and second dynamic pressure surfaces;
wherein the bearing member consists of a product formed by cold working austenitic stainless steel blank, and having a surface hardness of 300 Hv or harder obtained under a condition that the austenitic stainless steel blank is cold worked at a cold working ratio of 20% or higher.
In the present invention, the shaft member made of the flexible, austenitic stainless steel is cold worked, and in the cold working process, a cold working ratio is increased to a predetermined value or higher, whereby a surface of the shaft member is hardened. Therefore, there is no need for a nitriding treatment for the surface hardening. Corrosion proof degradation of the shaft member is prevented while leaving a passive coating inherently possessed by the stainless steel as intact. The hardening action based on the increase of the cold working ratio reaches to a depth of the shaft member deeper than by the conventional nitriding treatment. Therefore, formation of indentations in the shaft member surface when hit by something are also prevented more satisfactorily.
In this case, if nitrogen in the amount of 0.1 wet % or higher is added to the austenitic stainless steel blank, the surface hardness of the shaft member is further increased. The hardening process on the basis of the increased cold working ratio, which is applied to the shaft member is carried out so that the shaft member is hardened over a range of from the surface of the shaft member to a depth of 2 mm or deeper, the flaw formation is more reliably prevented.